1. Field of the Invention
This invention relates to making permanent electrical connection between a set of conductive tracks on a substrate and a co-operating set on a printed circuit. A particular application of this invention is in the making of edge connection to display devices such as liquid crystal displays.
2. Description of the Prior Art
Liquid crystal displays need some kind of connector to make contact with terminal pads on one or perhaps both of the substrates defining the liquid crystal layer. Usually these pads are of the same material as the electrodes of the display and are transparent, typically being made of indium tin oxide. A display cell may typically have between 30 and 400 such pads. Provided that they are not too numerous, and that they are not too fine, connections with these pads can sometimes be made using push-on connector sockets similar to those developed for making edge connection with printed circuit board. However, it is often found that this approach is unsatisfactory because of the frequency with which open circuit faults occur; moreover, there is the added disadvantage that the sockets tend to be relatively bulky. Finally, it is not really practical when the pitch of the pads is significantly less than 1 mm.
An alternative approach, particularly when the pads are relatively numerous and closely spaced, has been to make electrical connection between the pads and a co-operating set of conductive tracks on a piece of printed circuit by means of an elastomeric strip which is electrically non-conductive in the direction of the longitudinal axis of the strip, but is electrically conductive in planes normal to this axis. Such strip may be constructed by forming a laminate of alternate layers of electrically conductive and electrically insulating elastomeric material, with the layers extending across the strip so that the conduction of electricity in the longitudinal direction is prevented while that in transverse direction is permitted. This approach suffers from the disadvantage that the laminated elastomeric strip is relatively expensive and requires the use of some form of permanent clamping means to hold the strip in permanent compression. Such clamping means tends to be relatively bulky. The use of a fibre loaded adhesive is described in the specification of U.K. patent application No. 2 034 095 A, in which carbon fibres embedded in an adhesive matrix material are used not only to provide electrical connection between electrodes on the front and rear surfaces of a liquid crystal display cell, but also to provide spacers that will determine the spacing between the substrates carrying those electrodes; and hence, set a precise value to the thickness of the liquid crystal layer.